Tag: dark energy

I am very pleased to write that the Nobel Prize for physics this year has been awarded to three astronomers for their discovery of dark energy — a still-mysterious phenomenon that is causing the expansion of the Universe to accelerate.

Saul Perlmutter, Brian Schmidt, and Adam Riess are sharing the award. Back in 1998, Saul and Brian headed up two rival teams trying to observe very distant exploding stars, hoping they would yield better numbers for how fast the Universe expanded. Adam was on Brian’s team, and led the work on finding a way to try to understand the behavior of the supernovae. To everyone’s surprise, the data indicated the Universe was not just expanding, but expanding faster every day — it was accelerating.

Something must be pushing on the very fabric of space itself, causing it to expand ever-faster. We don’t now what it is, exactly, but we call it dark energy, and over the past 12 years, more and more observations have piled up showing that this stuff really is out there.

If you want background info on all this, see the Related Posts section below; there are plenty of links to articles I’ve written on this topic. The folks at Hubble also created a video describing dark energy and what it means for the Universe.

This is very exciting for lots of reasons. First, of course, it’s nice to see an astronomical topic win what is considered the top prize in science. Second, because I predicted it would years ago (not that this was all that difficult to see coming!). And third, for personal reasons, because I know all three of these men. I worked with Brian and Adam back in the day; the project Brian headed up to observe distant supernovae was part of a project using Hubble to observe supernovae in general, and I worked on a different aspect of it. Adam and I were both grad students at the time; after I got my PhD I went to work on a different Hubble project, and Adam stayed with the team, cracked the supernova code, and now has a Nobel Prize.

In 1998, two teams of astronomers independently reported amazing and bizarre news: the Universal expansion known for decades was not slowing down as expected, but was speeding up. Something was accelerating the Universe.

Since then, the existence of this something was fiercely debated, but time after time it fought with and overcame objections. Almost all professional astronomers now accept it’s real, but we still don’t know what the heck is causing it. So scientists keep going back to the telescopes and try to figure it out.

This gorgeous image is of the nearby spiral galaxy NGC 5584, where of course "nearby" to an astronomer means 72 million light years. This galaxy is loaded with a specific type of variable star — called Cepheids — which are very important: the way they change their brightness depends on how luminous they are. Measure the change, and you measure the luminosity, and if you measure how bright they appear in the sky you get their distance. It’s a bit like judging how far away a car is by gauging how bright its headlights are. Except in this case astronomers use Hubble instead of their eyes. It’s a tad more accurate.

It so happens that in 2007, NGC 5584 was the host of a Type Ia supernova, the Golden Standard of distance indicators. These are so bright they can be seen clear across the Universe! By knowing the distance to the one in NGC 5584, we can then use that to get the distances to supernovae much, much farther away.

Today in the U.S. is (only semi-jokingly) called Black Friday, because it’s traditionally a big shopping day for the holidays, which means crowds and madness.

So I was thinking about the term, which made me think about the adjective: black. What does it mean?

When I was a kid we’d argue if black was a color or not. Of course it is, some kids would say: there’s a crayon called black, and that is — for a kid — clearly the definitive source of evidence. But then someone would point out that black is the lack of color, the lack of light.

That’s correct. Black isn’t a color any more than male pattern baldness is a hairstyle. The lack of something usually isn’t something. I’ll leave it to you to argue over whether 0 is a number*.

So if black is the lack of light, can something every truly be black?

That’s an interesting question. First, I’ll note that in science there is this thing called a blackbody: an object like this would absorb all radiation that hits it, and warms up. As it warms up, it actually re-emits the absorbed energy in a specific way. Most of the energy is emitted at a certain wavelength of light depending on the temperature (the hotter an object is, the shorter the wavelength of the peak; see the plot shown here or read about Wien’s Displacement Law), with some given off at shorter and longer wavelengths. The graph of this looks a bit like an off-kilter bell curve. This idea is a very powerful one, since many objects behave in a manner pretty close to blackbodies: stars, planets, your oven top, and even you (your peak wavelength is well out into the infrared, at about 10μ roughly 12 times the reddest wavelength your eye can see).

So, paradoxically, to a scientist something that is black actually does emit light and color. But scientists are weird, and we’ll leave them to their theories. What about everyday folk? If I wanted to point out something really black to them, could I?

I think the answer is no. Look at it this way: what’s the blackest thing you know?Read More

Type Ia supernovae are very important exploding stars. It’s thought that this particular type of supernova has a very special property: they all explode with about the same energy. This makes them very valuable, because it means that if you can simply measure how bright they appear to be, you can figure out how far away they are. It’s like seeing headlights on the highway; dim ones are far away, and bright ones are close.

Of course, in reality, it’s not that easy. But after a Herculean effort, astronomers in the late 1990s figured they had been able to account for any small differences in brightness and could use these stars as "standard candles", benchmarks to calculate cosmic distances. Because they’re so bright, they make great milestones because they can be seen pretty much all the way to the edge of the observable Universe.

The thing is, it’s not clear how a type Ia actually forms. There are two models, both involving white dwarfs. These are the ultradense remnants of dead stars, the exposed cores of stars after they shed their outer layers. The Sun will one day be a white dwarf (in about 6 – 7 billion years, so don’t hold your breath). Because of complicated quantum physics, it turns out that white dwarfs can only have so much mass; if they exceed about 1.4 times the mass of the Sun they can collapse, either forming an even denser neutron star, or exploding as a supernova.

The first model of a Type Ia is a white dwarf orbiting a star like the Sun. The intense gravity of the dwarf draws material off the normal star, a process called accretion. The matter piles up, the mass limit is exceeded, and BANG! Supernova.

Well, it’s a lot more complicated than that, but close enough.

The second idea is that you have two white dwarfs orbiting each other. Over time they spiral in (this time due to relativistic effects called gravitational waves), get too close together, merge, and BANG! Supernova.

Astronomers have always assumed that the accretion scenario is the far more common of the two, because it takes a long time for two dwarfs to merge, whereas accretion can happen easily if a dwarf happens to be paired up with a normal star (which should be pretty common). But how do you tell which is which?

It turns out that the two different scenarios leading up to the explosion have two very different effects: accretion makes a lot of X-rays, while a merger does not. So astronomers did what you’d expect: they pointed the Chandra X-Ray Observatory at a bunch of galaxies and observed supernovae. What they found was pretty surprising: the amount of X-rays from Type Ia supernovae in nearby galaxies was 30 – 50 times lower than what would be expected from accretion. In other words, their observations strongly favor the idea that it’s the merger of white dwarfs that cause Type Ia supernovae.

Well! I was pretty surprised to hear that. Like other astronomers, I figured it was accretion that was the culprit. Now mind you, there are some caveats here. They observed elliptical galaxies, which tend to have an older population than spirals, so you might see more mergers than accretions. Also, it’s possible things were different in the past, and when we observe very distant galaxies were seeing them as they were billions of years ago.

But still, you just don’t expect to see what the astronomers saw, so it seems to me like they’re on to something here.

This has some interesting ramifications. It certainly affects a lot of fields of astronomy, like how binary stars form and change over time. But it may also affect cosmology, the study of the birth, evolution, and eventual fate of the Universe itself. If Type Ias are caused by a different scenario than previously thought, could it mean that our measurements of the distant Universe are wrong?

I asked this question specifically at the Chandra press conference, and was told that the two different scenarios produce explosions with pretty much the same energy, so this may only affect cosmological measurements a small amount. However, right now our theoretical models of the merger scenario are still pretty rough, so it’s unclear if the peak brightnesses of the two models are the same.

This may affect our measurements of dark energy, the mysterious pressure that seems to be accelerating the expansion of the Universe. My gut reaction is that this won’t matter a huge amount, since we have lots of independent ways of measuring dark energy, and they all appear to be in rough agreement. But this means we have one more thing to take into account in those measurements. And it may prove to be useful; if we can distinguish between the two supernova generators, our measurements will get that much more accurate.

I have to say I’m pleased with this; I studied supernovae in college and grad school, eventually studying one for my PhD (though it was of an entirely different flavor from this kind). I remember reading a long technical paper about the different Type Ia scenarios back then: it’s been a mystery for a long, long time. But with perseverance, the right equipment, and more than a touch of cleverness, we’re a big step closer to figuring this all out!

Whoa. That’s a part of a huge image just released by the Canada-France-Hawaii Telescope Legacy Survey Deep Field #1, a ginormous mosaic of the night sky… and by ginormous, I mean GINORMOUS. It covers a solid square degree of sky — 5 times the area of the full Moon — and tips the scale at a whopping 370 megapixels! It took 5 years and several hundred hours of observing time with the 3.6 meter telescope on top of Mauna Kea to get this massive mosaic.

The image itself may look cool and all, but the true power comes when you give in to the dark sideyou use the interactive zoom feature. You can surf the entire mammoth 370 million pixel image, zooming in on galaxies galore. And you won’t run out of objects to investigate any time soon: there are an estimated 500,000 galaxies in the image. Like the Hubble image I posted about yesterday, almost everything you see in the image above is a galaxy, not a star.

The images were taken to look for very distant supernovae. It was the investigation of these far-flung stellar explosions that led astronomers to determine the Universal expansion is accelerating, and to postulate the mysterious dark energy that powers this phenomenon. The CFHT is being used to map the same area of the sky over and over again, looking for the tell-tale blobs of light that mark the spots of a distant, dying suns. The more of these we see, the better we can nail down the physical characteristics of the cosmic expansion, and of the dark energy about which we know so little.

Of course, astronomers will squeeze a lot of science from this and other images… but it’s also OK to simply scan and pan through them at home, too, marveling that the Universe is so deep and so deeply beautiful.

The picture above shows a cosmic bulls-eye of epic alignment. But before I can tell you about it, I have to tell you about how the dart got thrown.

One of the more amazing aspects of looking into deep, deep space is that the path there is tortured and twisted. Space itself can be distorted by mass; it gets bent, like a road curves as it goes around a hill. And like a truck that must follow that road and steer around the hill, a photon must follow the curve of space.

Imagine a distant galaxy, billions of light years away. It emits light in all directions. One particular photon happens to be emitted almost — but not quite — in our direction. Left on its own, we’d never see it because it would miss the Earth by thousands or millions of light years.

But on its travels, it passes by another massive galaxy. This galaxy warps space, and the photon does what it must do: it follows that curve in pace, and changes direction… and it just so happens that the curve is just right to send it our way.

The intervening galaxy is essentially acting like a lens, bending the light. If the more distant galaxy is exactly behind the lensing galaxy, we see the light from that more distant galaxy distorted into a perfect ring, a circle of light surrounding the lens. We call this an Einstein Ring. If the farther galaxy is off to the side a bit, we see an arc instead of a complete ring. Gravitationally lensed arcs and rings are seen all over the sky, and they can be used to determine the mass of the intervening galaxy! The more mass, the more distorted the light from the farther galaxy. So the Universe has given us a nice method to let us weigh it.

In a surprising twist, astronomers have found a new type of lensed galaxy: a double ring! In a rare alignment, there are two distant galaxies aligned behind an intervening lensing galaxy. They’re like beads on a wire, lined up just right such that both more distant galaxies are lensed by the nearer one. In this case, the lens is about 3 billion light years away, and the other two are 6 and 11 billion light years away, an incredible distance.

This image is amazing, but it is also a powerful scientific tool. It allows us to measure not just the mass of the lensing galaxy, but also the amount of mysterious dark matter nearby. We cannot see the dark matter, but it too bends light, and contributes to the lensings. By observing lenses like this, we can take a sample of dark matter in the Universe, and that’s a crucial first step in understanding it. Even better, these double rings allows us to measure the amount of total mass not just in the nearest galaxy, as is usual, but also in the middle galaxy as well, since it distorts the light from the galaxy behind it (turns out it’s a rather lightweight one billion solar masses; our own Galaxy has more than 100 times that mass, so the middle galaxy is considered a dwarf).

This is a beautiful happenstance; it gives us a measure of the Universe at two points, with one being for free. In fact, Tommaso Treu, the astronomer at U.C. Santa Barbara who investigated this lens, points out that if we can find as few as 50 of these double rings, we can get a much better idea of the distribution of not just dark matter, but also the even more mysterious dark energy in the Universe. That’s one of the biggest goals of modern astronomy… and we may get a handle on it due to a coincidental ring toss.